Most anti-lock brake systems (ABS) cycle the brake fluid pressure to brake actuators to ascertain that the highest level of the tire to road adhesion is utilized. To cycle the brake fluid pressure to the brake actuators, the prior art, such as taught by Wetzel in U.S. Pat. No. 5,131,729 or in Leiber, U.S. Pat. No. 3,980,346, use a separate hydraulic pump driven by an electric motor. In U.S. Pat. No. 4,673,226, Every et al. teaches an anti-lock brake system in which only the driven wheels are controlled in an anti-lock mode, and the non-driven wheels are braked in a conventional manner. The transition of the driven wheels from a low friction (low mu) road surface to a high friction (high mu) road surface is detected by a decrease in the rotational speed of the driven wheels caused by the normally braked non-driven wheels engaging the high friction road surface which abruptly slows the vehicle's speed. However, the application of the anti-lock brake system taught by Every et al. to all four wheels of the vehicle has been inhibited by a means for detecting the transition of the wheels being braked from a low friction to a high friction surface.
The invention described herein allows for the detection of the transition of the wheels being braked from a low friction to a high friction surface when the braking system is in an anti-lock mode of operation.